After the casing broken, under the action of internal and external pressure, the sidewall rock surrounding the fracture occurred damage and fragmentation and entered into the wellbore by the fracture, accumulation in the borehole. Due to the different sizes of rock mass damage fragmentation, formulation program for milling and sandwashing in workover was largely affected, the paper was started from velocity of elastic longitudinal wave, based on probability theory and the application of fractal geometry theory, a new calculation method of rock fractal dimension was established. The traditional theory of rock mass damage fragment distribution was enriched. The rock mass damage fragment distribution was more easily and more efficiently forecasted. And the reference frame of parameter design for casing-damaged well work over was provided, and guiding site-operation was quickly and effectively constructed.